The present invention relates to the field of technology of gas turbines. It concerns a heat shield for a gas turbine, which heat shield encloses in an annular manner the moving blades, rotating in the hot-gas duct of the gas turbine, of a stage of the gas turbine and consists of a plurality of heat-shield segments which are arranged one behind the other in the circumferential direction, are curved in the shape of a segment of a circle and are cooled from outside, and the longitudinal sides of which are designed as correspondingly curved rails (which may be either continuous or discontinuous) running in the circumferential direction and having in each case a pair of arms which project in the axial direction, run in parallel and are at a distance from one another, the heat-shield segments, while forming a cavity to which cooling air can be admitted, are fastened to the inside of an annular carrier, which concentrically surrounds the heat shield in such a way that in each case a radial gap is formed between the longitudinal sides of the heat-shield segments and the adjacent elements which define the hot-gas duct on the outside.
Such a heat shield has been disclosed, for example, by the publications U.S. Pat. Nos. 4,177,004, 4,551,064, 5,071,313, 5,584,651 or EP-A1-0 516 322.
Heat shields for gas turbines, which surround the moving blades of a turbine stage in an annular manner and, on the one hand, define the hot-gas duct on the outside and, on the other hand, keep the gap between the outer wall of the hot-gas duct and the ends of the moving blades as small as possible for reasons of efficiency without causing abrasive contact during fluctuating temperatures, have been known for a long time. Such heat shields normally consist of a multiplicity of heat-shield segments which are curved in the shape of a segment of a circle and, arranged one behind the other in the circumferential direction, form a closed ring.
The individual heat-shield segments are often detachably fastened to a carrier, which concentrically surrounds the heat shield. In this case, on account of the different thermal expansion of the various individual parts, care is taken to ensure that radial gaps or annular-gap-shaped cavities remain free between the heat-shield segments and the adjacent elements which define the hot-gas duct on the outside.
The heat shield or the individual heat-shield segments are subjected to high thermal loading during operation of the gas turbine. One the one hand, this thermal loading may have adverse effects on the heat shield itself. On the other hand, the heat may be conducted outward through the shield and cause damage there. Measures are therefore normally taken in order to suitably cool the heat-shield segments from the rear or outside by compressed cooling air, which usually originates from the compressor part of the gas turbine or the plenum. This cooling is to be as even and as efficient as possible and is to include all the loaded regions of the heat shield. In addition, hot gas should be prevented from penetrating into the adjacent gaps in the outer wall of the hot-gas duct and undesirably heating the parts of the construction which lie behind it.
A heat shield for a gas turbine is disclosed in U.S. Pat. No. 4,177,004 (FIGS. 1, 2 and 4 there), in which cooling air is fed from the cavity (52) lying behind the heat-shield segments through cooling holes (66) into the adjacent intermediate space (48) only on the downstream longitudinal side of the heat-shield segments and is directed from the intermediate space (48) through cooling slots (67) in the clamp part (43) into the hot-gas duct (FIG. 4, FIG. 5). In contrast, cooling air is only passed externally around the upstream longitudinal side of the heat-shield segments (FIG. 3) and flows in other ways into the cavity (62) lying behind them. This arrangement has the disadvantage that the heat-shield segment as a whole is cooled unevenly, since cooling from the rear virtually does not take place on that longitudinal side of the heat-shield segment which is oriented upstream. A further disadvantage is that the cooling slots (67) have been made in the clamp element (43), which in terms of manufacture leads to a considerable extra cost.
In the solution described in U.S. Pat. No. 4,551,064, (angled) cooling holes (55) are also arranged only in the region of the downstream longitudinal edge of the heat-shield segment. Both gaps (64, 68) adjacent to the heat-shield segments are flooded by cooling-air flows (59 and 65 resp. in FIG. 1) which are fed through separate holes (63, 67) from outside the heat shield.
Disclosed in U.S. Pat. No. 5,584,651 is a heat shield in whose segments an inner cavity (38) is formed in the upstream edge (FIG. 2), and the cooling air flows through this cavity (38) and discharges through outlet holes (44), arranged directly at the edge, into the hot-gas duct. On the other hand, in the marginal region located downstream or in the region of the segments there, no special cooling is provided, so that very uneven cooling of the heat-shield segments can be expected in this case too. The downstream inner arms of the heat-shield segments with the edges (28b in FIG. 1) are especially affected by this.
Somewhat more extensive cooling is achieved by the cooling holes (80) extending further downstream in the heat shield from EP-Al-0 516 322. Here too, however, the downstream longitudinal edge of the heat shields with the inner arms (44) is virtually uncooled.
The object of the invention is therefore to provide a heat shield for a gas turbine, which heat shield avoids the disadvantages of known heat shields and, with at the same time a simple construction, is distinguished by efficient and even cooling over the entire thermally loaded area of the heat-shield segments and in particular of the inner arms projecting axially on the longitudinal edges.
The object is achieved by all the features of claim 1. The essence of the invention consists in directing cooling air from the cavity lying behind the segments through corresponding cooling holes into the adjacent gaps at both longitudinal sides of the heat shields, that is, both upstream and downstream, and thus in also simultaneously and evenly cooling the two longitudinal-edge regions of the heat-shield segments and flooding the gaps to prevent an ingress of hot gases. In this case, all the cooling and flooding features are arranged (in the form of cooling holes or cooling slots) on the heat-shield segment itself, which substantially facilitates the manufacture and makes it unnecessary to adapt the other parts of the heat-shield duct. The outflow of the cooling air at both longitudinal sides of the heat-shield segments also results in the cooling air sweeping more evenly over the outsides, defining the cavity, of the segments and thus evenly cooling the entire segment area. As a result, the thermal loading over the entire area is evenly reduced and the service life of the heat-shield segments is significantly prolonged.
A first preferred embodiment of the heat shield according to the invention is characterized in that the heat-shield segments are fastened to the carrier by means of clamps, which, with ends bent inward in an L-shape, engage from both sides under the carrier in the intermediate spaces formed between the arm pairs, in that the cooling air flowing out of the cooling holes is directed in the intermediate spaces between the ends, bent inward in an L-shape, of the clamps and the inner arms of the heat-shield segments to the gaps, and in that cooling slots in alignment with the cooling holes are made in the outsides of the inner arms in order to direct the cooling air discharging from the cooling holes. Due to the cooling slots in the inner arms, the heat-transfer area at the arms is increased and the cooling of the arms (furthest away from the cavity filled with cooling air) is substantially evened out and improved.
A second preferred embodiment of the heat shield according to the invention is characterized in that, to reduce the deflection of the heat shield during temperature changes, axially running stiffening ribs are arranged or integrally formed on the outside of the heat-shield segments in the region of the cavity, in that an impingement-cooling plate running in the circumferential direction and provided with openings is arranged inside the cavity and at a distance from the outside of the heat-shield segments, and in that individual lugs or pins, which project radially outward and on which the impingement-cooling plate is supported, are arranged inside the stiffening ribs. The stiffening ribs with the formed lugs stiffen the heat-shield segments in the axial direction and thereby reduce the risk of the moving blades grazing against the heat shield. In addition, they improve the heat transfer between the segment and the cooling air flowing through the cavity. In this case, the lugs, which serve to support the impingement-cooling plate, may be formed together with the stiffening ribs in a simple manner during the casting of the segments.
The cooling air is effectively prevented from flowing off undesirably from the gaps if, in a further preferred embodiment of the invention, first axial elastic seals are arranged above the cooling holes between the clamps and the longitudinal sides of the heat-shield segments.
Further embodiments follow from the dependent claims.